The invention pertains to the field of molecular biology. In particular, the invention pertains to genes which confer resistance on plants to Verticillium species.
Verticillium wilt is a common vascular disease that causes severe yield and quality losses in many crops. The disease is caused by fungi of the genus Verticillium, most commonly Verticillium albo-atrum Reingke and Berthier or Verticillium dahliae Kleb. The relationship between V. albo-atrum and V. dahliae has been the subject of debate (see, generally, Domsch et al., 1980). They have previously been considered representatives of a single variable species, but they have more recently been viewed as distinct species.
V. albo-atrum was first identified as a causal agent of Verticillium wilt in potato, but is now known also to cause wilt in hop, tomato, alfalfa, strawberry, sainfoin, runner bean, broad bean, pea, clover and cucumber. Vascular infection of these hosts by V. albo-atrum leads to wilt, with or without obvious flaccidity, and, commonly, browning of the infected xylem stems.
V. dahlia is far more common than V. albo-atrum, but causes disease symptoms that are less severe than those caused by V. albo-atrum infection. Further, in the above-mentioned hosts, V. dahlia is usually less virulent than V. albo-atrum. V. dahlia is known to infect canola, cotton, dahlia, mint species, vine, tomato, potato, eggplant, olive, pistachio, stone fruits, Brussels sprouts, groundnut, horse radish, tobacco, red pepper, strawberry, and other plant species. Symptoms of V. dahlia infection usually include flaccidity or chlorosis of leaves, followed by permanent wilting.
In a recent survey in North America, Verticillium wilt was ranked as the most important disease of both seed and commercial potato crops, and the second greatest constraint on tuber yield. Pathogen-mediated reductions in net photosynthesis, transpiration, and increased leaf temperature cause premature foliage senescence and yield loss. Disease symptoms in potato include wilting and leaf chlorosis and necrosis, while the tubers of infected plants develop necrosis in the vascular tissue that reduces tuber quality, in particular for the manufacture of french fries and chips. Abiotic factors such as moisture stress and high temperatures accelerate development of visual disease symptoms. Studies have demonstrated a synergistic interaction between root-lesion nematodes (Pratylenchus penetrans) and Verticillium wilt, further complicating disease control.
Management strategies for the control of Verticillium wilt include soil fumigation, crop rotation, and development of resistant cultivars. Of these strategies, only disease resistance is effective. While several recent potato cultivar releases have some resistance to the pathogen, the major potato varieties grown in North America are susceptible. The genetic mechanisms of resistance towards fungi of the taxon Verticillium spp. vary from polygenic in strawberry and alfalfa, to a dominant single gene in cotton, sunflower, and tomato. Conclusions from studies with tetraploid Solanum tuberosum subsp. tuberosum L. suggest that inheritance is polygenic and complex. Screening for resistance in the non-cultivated diploid tuber-bearing wild Solanum species has identified resistance but not immunity, and a recent study concluded that resistance is polygenic and genetically complex. Thus, incorporation of resistance into new potato cultivars by classical breeding techniques is difficult and inefficient. It would therefore be desirable to develop alternative approaches to obtaining plants that are resistant to Verticillium wilt.
Plants have natural defenses which prevent infection of the plant by viruses, bacteria, fungi, nematodes and insects. As plants do not have a circulatory system, each plant cell must have a preformed or inducible defensive capacity. Recently, disease resistance genes, which confer resistance to specific pathogens, have been identified in various plants. It is believed that the mechanism of resistance may differ depending on the mode of pathogen attack.
Necrotrophy, biotrophy and hemibiotrophy are the three principal modes of pathogen attack on plants (see, generally, Hammond-Kosack et al., 1997). Necrotrophs first kill host cells, then metabolize the cell contents. Necrotrophs often have a broad host range, and cause host cell death with toxins or enzymes targeted to certain substrates. Plant resistance to necrotrophs may therefore be achieved through loss or alteration of the target of the toxin in the plant, or through detoxification of the toxin produced by the plant. An example of a plant disease resistance gene of the latter type Hm1, isolated from maize, and which confers resistance to the leaf spot fungus Cochliobolus carbonum. Hm1 encodes a reductase enzyme which is thought to inactivate the C. carbonum toxin.
Flor (1949) developed the classic xe2x80x9cgene-for-genexe2x80x9d model for plant pathogen resistance. Flor proposed that for incompatibility (i.e. resistance) to occur, complementary pairs of dominant genes are required, one in the host and the other in the pathogen. Loss or alteration of either the plant resistance (xe2x80x9cRxe2x80x9d) gene or the pathogen avirulence (xe2x80x9cAvrxe2x80x9d) gene results in compatibility (i.e. disease). It is thought that R genes encode proteins that can recognize Avr-gene-dependent ligands. The simplest possibility is that the Avr-gene-dependent ligand binds directly to the R gene product. Following binding, the R gene product is believed to activate downstream signaling cascades to induce defense responses, such as the hypersensitive response, which causes localized plant cell death at the point of pathogen attack, thereby depriving the pathogen of living host cells. Downstream signaling components may include kinase and phosphatase cascades, transcription factors, and reactive oxygen species (see, generally, Hammond-Kosack et al., 1997).
Hammond-Kosack et al. (1997) summarize the five classes of known R genes, classified according to predicted features of the R gene product.
The first class is composed of R genes that encode detoxifying enzymes. An example is Hm1 from maize, discussed above, which confers resistance to Cochliobolus carbonum (Johal et al., 1992).
R genes of the second class encode intracellular serine/threonine-specific protein kinases. An example is Pto, isolated from tomato, and which confers resistance to Pseudomonas syringe pv. tomato (Martin et al., 1993).
The third class of R genes is divided into two subclasses. The first encode intracellular proteins with an amino terminal leucine zipper domain, a nucleotide binding site (xe2x80x9cNBSxe2x80x9d) domain, and a leucine rich repeat (xe2x80x9cLRRxe2x80x9d) domain. Examples include RPS2 of Arabidopsis, which confers resistance to Pseudomonas syringe pv. tomato (Bent et al., 1994; Mindrinos et al., 1994), and I2, from tomato, which confers resistance to Fusarium oxysporum (Ori et al., 1997). R genes of the second subclass encode intracellular proteins with an amino terminal domain having homology with Drosophila Toll protein, and NBS and LRR domains. Examples include R gene N of the tobacco plant, which confers resistance to tobacco mosaic virus (Whitman et al., 1994), and L6 of flax, which confers resistance to Melampsora lini (Lawrence et al., 1995).
R genes of the fourth class encode proteins having an extracellular LRR, a single membrane spanning region, and a short cytoplasmic carboxyl terminus. An example is Cf9 of tomato, which confers resistance to Cladosporium fulvum (Jones et al., 1994).
The fifth class encompasses R genes encoding proteins having an extracellular LRR, a single membrane spanning region, and a cytoplasmic kinase domain. An example is R gene Xa21 of rice, which confers resistance to Xanthomonas oryzae pv. oryzae (Song et al., 1995).
Certain isolated R genes have been introduced into susceptible plants, resulting in transgenic, disease-resistant plants. For instance, U.S. Pat. No. 5,859,339 teaches transformation of susceptible rice plants with the rice Xa21R gene, resulting in transgenic rice plants that are resistant to Xanthomonas infection. U.S. Pat. No. 5,920,000 teaches transformation of susceptible tomato plants with the tomato Cf9 R gene, resulting in transgenic tomato plants that are resistant to infection by Cladosporium fulvum. 
However, the prior art R genes and methods do not confer on plants resistance to Verticillium species. Hence, there is a need for isolated Verticillium wilt resistance genes, and for methods for conferring resistance on plant species to infection by Verticillium species.
The invention provides isolated polynucleotides of at least about 50 nucleotides, preferably at least about 100 nucleotides, more preferably at least about 200 nucleotides, more preferably at least about 500 nucleotides, more preferably at least about 1000 nucleotides, and even more preferably at least about 2000 nucleotides, which encode polypeptides comprising amino acid sequences having at least 40% homology to the amino acid sequence depicted in SEQ ID NO: 2, 4, 6, 8, or 14 and which polynucleotides, when present in a plant, confer on the plant resistance to Verticillium species. These polynucleotides are denoted herein as xe2x80x9cVexe2x80x9d polynucleotides. The isolated Ve polynucleotides preferably encode polypeptides comprising amino acid sequences having at least 50% homology, more preferably at least 60% homology, more preferably at least 70% homology, more preferably at least 80% homology, more preferably at least 85% homology, more preferably at least 90% homology, and even more preferably at least 95% homology with the amino acid sequence depicted in SEQ ID NO: 2, 4, 6, 8, or 14. Preferably, the polynucleotides of the invention are isolated from tomato (Lycopersicon esculentum), though they may be obtained from other plant species such as potato, hop, alfalfa, strawberry, sainfoin, runner bean, broad bean, pea, clover, cucumber, canola, cotton, dahlia, mint species, vine, eggplant, olive, pistachio, stone fruit, Brussels sprouts, groundnut, horse radish, tobacco, and red pepper.
In an exemplified case, the Ve polynucleotide is isolated from L. esculentum, and has the nucleotide sequence depicted in SEQ ID NO: 1 from nucleotide 1 to nucleotide 3417, SEQ ID NO: 3 from nucleotide 57 to nucleotide 3473, SEQ ID NO: 5 from nucleotide 1 to nucleotide 3159, or in SEQ ID NO: 7 from nucleotide 1 to nucleotide 3159. The sequences depicted in SEQ ID NO: 3 from nucleotide 57 to nucleotide 3473, and in SEQ ID NO: 7 from nucleotide 1 to nucleotide 3159 are complementary DNA (xe2x80x9ccDNAxe2x80x9d) sequences, and are denoted herein as, respectively, the L. esculentum verticillium wilt resistance genes Ve1.1 and Ve1.2. The sequences depicted in SEQ ID NO: 1 from nucleotide 1 to nucleotide 3417, and SEQ ID NO: 5 from nucleotide 1 to nucleotide 3159 are genomic DNA sequences of Ve1.1 and Ve1.2. Ve1.1 and Ve1.2 are two open reading frames (xe2x80x9cORFsxe2x80x9d) occurring at the same locus, denoted herein as Ve1.
In another exemplified case, the Ve polynucleotide is isolated from Solanum chacoense, has the partial nucleotide sequence depicted in SEQ ID NO: 13, and is denoted herein as Vc.
The invention extends to purified and isolated polynucleotides of at least 50 nucleotides, preferably at least 100 nucleotides, more preferably at least 200 nucleotides, even more preferably at least 500 nucleotides, and most preferably at least 1000 nucleotides, comprising Verticillium wilt resistance gene promoters. An exemplified promoter sequence is depicted in SEQ ID NO: 10.
The invention also provides nucleic acid constructs, vectors, and transformed cells containing at least one of the aforementioned Ve polynucleotides.
The invention further extends to transgenic plants, cells, seeds and embryos transformed with at least one Ve polynucleotide of the invention, and to methods for conferring on susceptible plants resistance to Verticillium species, by transforming plant cells with at least one Ve polynucleotide of the invention and regenerating mature plants. It may be desirable to transform the plant with more than one Ve polynucleotide. For instance, an additive effect may result if the plant is transformed with both Ve1.1 and Ve1.2.
The invention further extends to methods for obtaining a polynucleotide which, when present in a plant, confers on the plant resistance to Verticillium species. Broadly stated, such methods comprise the steps of: isolating polynucleotides from a plant having resistance to Verticillium species; performing nucleic acid hybridization between said polynucleotides and a probe comprising a nucleotide sequence derived from any of the sequences depicted in SEQ ID NO:1-8, 10, 13 or 14; and, testing polynucleotides that hybridize with said probe for ability to confer on a plant resistance to Verticillium species.
FIG. 1 is a schematic genetic and physical representation of the L. esculentum linkage group surrounding Ve1. Analysis of populations segregating for Ve1 identified closely linked co-dominant random amplified polymorphic DNAs (xe2x80x9cRAPDsxe2x80x9d) and allele-specific sequence characterized amplified regions (xe2x80x9cSCARsxe2x80x9d) that map to the region of restriction fragment length polymorphism (xe2x80x9cRFLPxe2x80x9d) GP39. Identification of contiguous xcex genomic clones facilitated the subcloning of genomic DNA containing the Ve1 locus. Vertical lines indicate location of the AUG initiation codons. Expressed sequences were cloned into xcex and the arrowhead depicts the direction of transcription for the cDNA identified using the genomic clone pG1Ve. Potato plants transformed with the genomic subclones (pG1Ve, pG2Ve, pG3Ve) and cDNA (pC1.1Ve, pC1.2Ve) exhibited in vivo complementation and resistance (R) when challenged with Verticillium albo-atrum. 
FIGS. 2A and 2B depict the Ve1.1 genomic DNA sequence (SEQ ID NO: 1, from nucleotide 1 to nucleotide 3417). The deduced Ve1.1 amino acid sequence encoded by the Ve1.1 genomic DNA sequence is depicted in SEQ ID NO: 2.
FIGS. 3A and 3B depict the Ve1.1 cDNA sequence (SEQ ID NO: 3 from nucleotide 57 to nucleotide 3473).
FIGS. 4A and 4B depict the primary structure of the encoded Ve1.1 protein (SEQ ID NO: 4) deduced from the Ve1.1 cDNA sequence (SEQ ID NO: 3 from nucleotide 57 to nucleotide 3473). Domains A through F are identified, as described in Example 1 herein. In the potential membrane associated domains A and C, hydrophobic amino acids are underlined. Within the leucine-rich repeat (xe2x80x9cLRRxe2x80x9d) domain B, conserved amino acids L and G, and potential N-glycosylation sites, are underlined. In domain D, the neutral and basic amino acids are underlined. In domain E, acidic amino acids of the putative PEST sequence are underlined.
FIGS. 5A and 5B depict the Ve1.2 genomic DNA sequence (SEQ ID NO: 5 from nucleotide 1 to nucleotide 3159). The deduced Ve1.2 amino acid sequence encoded by the Ve1.2 genomic DNA sequence is depicted in SEQ ID NO: 6.
FIGS. 6A and 6B depict the Ve1.2 cDNA sequence (SEQ ID NO: 7 from nucleotide 1 to nucleotide 3159).
FIG. 7 depicts the primary structure of the encoded Ve1.2 protein (SEQ ID NO: 8) deduced from the Ve1.2 cDNA sequence (SEQ ID NO: 7 from nucleotide 1 to nucleotide 3159). Domains A through E are identified, as described in Example 1 herein. In the potential membrane associated domains A and C, hydrophobic amino acids are underlined. The italicized amino acids in domain A represent the putative leucine zipper region. Within the LRR domain B, conserved amino acids L and G, and potential N-glycosylation sites are underlined. In domain D, the neutral and basic amino acids are underlined. In domain E, acidic amino acids of the putative PEST sequence are underlined.
FIG. 8 is a Northern analysis of Ve1.1 cDNA expression. Hybridization of pC1Ve was determined using 5 xcexcg of poly (A+) RNA extracted from uninoculated (A) Ailsa Craig and (B) Craigella or (C) Ailsa Craig and (D) Craigella inoculated with race 1 of V. dahliae three days prior to RNA isolation. Transcript of Ve1 is constitutively expressed in susceptible and resistant genotypes.
FIGS. 9A and 9B are a sequence alignment of the Ve1.1 and Ve1.2 protein sequences. The sequences have about 84% identity. Aligned identical residues are identified by the xe2x80x9c|xe2x80x9d character. Aligned similar residues are identified by the xe2x80x9c.xe2x80x9d character. Similar amino acids are deemed to be: A, S, and T; D and E; N and Q; R and K; I, L, M and V; F, Y and W.
FIG. 10 depicts a partial genomic DNA sequence of the Vc Verticillium wilt resistance gene of Solanum chacoense (SEQ ID NO: 13). The deduced partial Vc amino acid sequence encoded by the Vc genomic DNA sequence is depicted in SEQ ID NO: 14.
FIGS. 11A and 11B are a sequence alignment of the Ve1.2 protein sequence and the Vc partial protein sequence. The sequences have about 72% identity. Aligned identical residues are identified by the xe2x80x9c|xe2x80x9d character. Aligned similar residues are identified by the xe2x80x9c.xe2x80x9d character. Similar amino acids are deemed to be: A, S, and T; D and E; N and Q; R and K; I, L, M and V; F, Y and W.
In order to provide a clear and consistent understanding of the specification and claims, including the scope to be given to such terms, the following definitions are provided.
A polynucleotide or polypeptide having the xe2x80x9cbiological activityxe2x80x9d of, respectively, an exemplified Ve polynucleotide or Ve polypeptide of the invention, is a sequence that is functional in a plant to confer resistance to Verticillium species, whether to a greater or lesser degree than the subject Ve polynucleotide or Ve polypeptide.
A xe2x80x9ccoding sequencexe2x80x9d is the part of a gene which codes for the amino acid sequence of a protein, or for a functional RNA such as a tRNA or rRNA.
A xe2x80x9ccomplementxe2x80x9d or xe2x80x9ccomplementary sequencexe2x80x9d is a sequence of nucleotides which forms a hydrogen-bonded duplex with another sequence of nucleotides according to Watson-Crick base-pairing rules. For example, the complementary base sequence for 5xe2x80x2-AAGGCT-3xe2x80x3 is 3xe2x80x2-TTCCGA-5xe2x80x2.
A xe2x80x9cdomainxe2x80x9d of a polypeptide is a portion or region of the polypeptide that forms a structural or functional niche within the remainder of the polypeptide. For example, DNA-binding proteins have DNA-binding domains with specific features such as helix-turn-helix configurations or Zn2+-fingers which enable them to recognize and bind to specific structures or sequences on their target DNA with high specificity and affinity. A xe2x80x9chydrophobicxe2x80x9d domain is a domain containing more amino acids having hydrophobic (nonpolar) R groups than amino acids having hydrophilic (polar but uncharged) R groups. A xe2x80x9cbasicxe2x80x9d domain is a domain containing more amino acids having basic R groups than amino acids having acidic R groups. An xe2x80x9cacidicxe2x80x9d domain is a domain containing more amino acids having acidic R groups than amino acids having basic R groups. The R groups of the 20 amino acids common in proteins are reviewed hereinafter. A domain comprising a plurality of xe2x80x9cleucine-rich repeatsxe2x80x9d is a domain containing more than one repeat of the 24 amino acid consensus sequence depicted in SEQ ID NO: 9.
xe2x80x9cDownstreamxe2x80x9d means on the 3xe2x80x2 side of any site in DNA or RNA.
xe2x80x9cExpressionxe2x80x9d refers to the transcription of a gene into structural RNA (rRNA, tRNA) or messenger RNA (mRNA) with subsequent translation into a protein.
Two polynucleotides are xe2x80x9cfunctionally equivalentxe2x80x9d if they perform substantially the same biological function. For instance, two polynucleotides are functionally equivalent Verticillium wilt resistance gene promoters if both are functional in a plant to effect the transcription of a Ve polynucleotide operably linked thereto.
Two nucleic acid sequences are xe2x80x9cheterologousxe2x80x9d to one another if the sequences are derived from separate organisms, whether or not such organisms are of different species, as long as the sequences do not naturally occur together in the same arrangement in the same organism.
Two polynucleotides or polypeptides are xe2x80x9chomologousxe2x80x9d or xe2x80x9cidenticalxe2x80x9d if the sequence of nucleotides or amino acid residues, respectively, in the two sequences is the same when aligned for maximum correspondence as described herein. Sequence comparisons between two or more polynucleotides or polypeptides are generally performed by comparing portions of the two sequences over a comparison window to identify and compare local regions of sequence similarity. The comparison window is generally from about 20 to about 200 contiguous nucleotides or contiguous amino acid residues. The xe2x80x9cpercentage of sequence identityxe2x80x9d or xe2x80x9cpercentage of sequence homologyxe2x80x9d for polynucleotides and polypeptides may be determined by comparing two optimally aligned sequences over a comparison window, wherein the portion of the polynucleotide or polypeptide sequence in the comparison window may include additions or deletions (i.e., gaps) as compared to the reference sequence (which does not comprise additions or deletions) for optimal alignment of the two sequences. The percentage is calculated by: (a) determining the number of positions at which the identical nucleic acid base or amino acid residue occurs in both sequences to yield the number of matched positions; (b) dividing the number of matched positions by the total number of positions in the window of comparison; and, (c) multiplying the result by 100 to yield the percentage of sequence identity.
Optimal alignment of sequences for comparison may be conducted by computerized implementations of known algorithms, or by inspection. A list providing sources of both commercial available and free software is found in Ausubel et al. (1999, and in previous editions). Readily available sequence comparison and multiple sequence alignment algorithms are, respectively, the Basic Local Alignment Search Tool (BLAST) (Altschul et al., 1990; Altschul et al., 1997) and ClustalW programs. Other suitable programs include GAP, BESTFIT, FASTA, and TFASTA in the Wisconsin Genetics Software Package (Genetics Group (GCG), 575 Science Dr., Madison, Wis.). For greater certainty, as used herein and in the claims, xe2x80x9cpercentage of sequence identityxe2x80x9d or xe2x80x9cpercentage of sequence homologyxe2x80x9d of amino acid sequences is determined based on optimal sequence alignments determined in accordance with the default values of the BLASTX program.
As discussed in greater detail hereinafter, homology between nucleotide sequences can also be determined by DNA hybridization analysis, wherein the stability of the double-stranded DNA hybrid is dependent on the extent of base pairing that occurs. Conditions of high temperature and/or low salt content reduce the stability of the hybrid, and can be varied to prevent annealing of sequences having less than a selected degree of homology.
xe2x80x9cIsolatedxe2x80x9d means altered xe2x80x9cby the hand of manxe2x80x9d from the natural state. If an xe2x80x9cisolatedxe2x80x9d composition or substance occurs in nature, it has been changed or removed from its original environment, or both. For example, a polynucleotide or a polypeptide naturally present in a living animal is not xe2x80x9cisolatedxe2x80x9d, but the same polynucleotide or polypeptide separated from the coexisting materials of its natural state is xe2x80x9cisolatedxe2x80x9d, as the term is employed herein.
Two DNA sequences are xe2x80x9coperably linkedxe2x80x9d if the nature of the linkage does not interfere with the ability of the sequences to effect their normal functions relative to each other. For instance, a promoter region would be operably linked to a coding sequence if the promoter were capable of effecting transcription of that coding sequence.
A xe2x80x9cpolynucleotidexe2x80x9d is a linear sequence of deoxyribonucleotides (in DNA) or ribonucleotides (in RNA) in which the 3xe2x80x2 carbon of the pentose sugar of one nucleotide is linked to the 5xe2x80x2 carbon of the pentose sugar of the adjacent nucleotide via a phosphate group.
A xe2x80x9cpolynucleotide constructxe2x80x9d is a nucleic acid molecule which is isolated from a naturally occurring gene or which has been modified to contain segments of nucleic acid which are combined and juxtaposed in a manner which would not otherwise exist in nature.
A xe2x80x9cpolypeptidexe2x80x9d is a linear polymer of amino acids that are linked by peptide bonds.
A xe2x80x9cpromoterxe2x80x9d is a cis-acting DNA sequence, generally 80-120 base pairs long and located upstream of the initiation site of a gene, to which RNA polymerase may bind and initiate correct transcription.
A xe2x80x9crecombinantxe2x80x9d polynucleotide, for instance a recombinant DNA molecule, is a novel nucleic acid sequence formed in vitro through the ligation of two or more nonhomologous DNA molecules (for example a recombinant plasmid containing one or more inserts of foreign DNA cloned into its cloning site or its polylinker).
A plant that is xe2x80x9cresistantxe2x80x9d to Verticillium species, is a plant that, after inoculation with a strain of Verticillium, such as Verticillium albo-atrum, exhibits reduced or delayed symptoms of Verticillium wilt, such as wilting, chlorosis, necrosis, and death, relative to a control plant inoculated with the pathogen under the same conditions. A control plant may be, for example, a plant of the same species that has been transformed with a vector that does not have a disease resistance gene inserted therein. Accordingly, a gene that confers xe2x80x9cresistancexe2x80x9d to Verticillium species is a gene that, when present in a plant, makes the plant resistant to Verticillium species.
A xe2x80x9cSolanaceous plantxe2x80x9d is a plant of the botanical family Solanaceae. Also known as the nightshade family, this group includes several widely cultivated plants such as potato, tomato, tobacco, pepper, eggplant, and petunia.
xe2x80x9cTransformationxe2x80x9d means the directed modification of the genome of a cell by the external application of purified recombinant DNA from another cell of different genotype, leading to its uptake and integration into the subject cell""s genome. In bacteria, the recombinant DNA is not integrated into the bacterial chromosome, but instead replicates autonomously as a plasmid.
A xe2x80x9ctransgenicxe2x80x9d organism, such as a transgenic plant, is an organism into which foreign DNA has been introduced. A xe2x80x9ctransgenic plantxe2x80x9d encompasses all descendants, hybrids, and crosses thereof, whether reproduced sexually or asexually, and which continue to harbour the foreign DNA.
xe2x80x9cUpstreamxe2x80x9d means on the 5xe2x80x2 side of any site in DNA or RNA.
A xe2x80x9cvectorxe2x80x9d is a nucleic acid molecule that is able to replicate autonomously in a host cell and can accept foreign DNA. A vector carries its own origin of replication, one or more unique recognition sites for restriction endonucleases which can be used for the insertion of foreign DNA, and usually selectable markers such as genes coding for antibiotic resistance, and often recognition sequences (e.g. promoter) for the expression of the inserted DNA. Common vectors include plasmid vectors and phage vectors.
A xe2x80x9cVe polynucleotidexe2x80x9d is a polynucleotide which, when present in a plant, including a transgenic plant, confers on the plant resistance to Verticillium species.
A xe2x80x9cVe polypeptidexe2x80x9d is a polypeptide encoded by a Ve polynucleotide, and which is active to confer resistance to Verticillium species on a plant.
xe2x80x9cVerticillium wiltxe2x80x9d is a plant disease caused by a fungus of the genus Verticillium, and which is characterized by wilting, chlorosis, necrosis, reduced yield, and, possibly, eventual death of the infected plant.
The term xe2x80x9cVerticillium speciesxe2x80x9d encompasses all members of the genus Verticillium, as described by Domsch et al. (1980). Verticillium species include, without limitation, V. albo-atrum, V dahlia, V. catenulatum, V chlamydosporium, V. lecanii, V nigrescens, V. nubilum, V. psalliotae, and V. tricorpus. 
The invention provides Ve polynucleotides which, when present in a plant, confer on the plant resistance to Verticillium species. In an exemplified case, the Ve polynucleotide is isolated from Lycopersicon esculentum, and has the cDNA sequence depicted in SEQ ID NO: 3 from nucleotide 57 to nucleotide 3473 (Ve1.1), or in SEQ ID NO: 7 from nucleotide 1 to nucleotide 3159 (Ve1.2) and encodes, respectively, the Ve polypeptide having the amino acid sequence depicted in SEQ ID NOS: 4 or 8. Although it appears that Ve1.1 and Ve1.2 do not contain introns, the respective genomic DNA sequences depicted in SEQ ID NO: 1 from nucleotide 1 to nucleotide 3417 and SEQ ID NO: 5 from nucleotide 1 to nucleotide 3159, and consequently the deduced amino acid sequences depicted in SEQ ID NOS: 2 and 6, differ slightly from the cDNA sequences. Without being limited by the same, it is thought that this difference may be due to natural variation among L. esculentum strains. Although both Ve1.1 and Ve1.2 are functional to confer on plants resistance to Verticillium species, as shown in FIG. 9, the Ve1.1 and Ve1.2 DNA sequences differ substantially, and have only about 84% sequence homology.
As described in detail in Example 1 herein, Ve1.1 and Ve1.2 were isolated from a strain of L. esculentum (tomato) that is resistant to Verticillium dahliae. Potato plants regenerated from plant cells transformed with a vector containing Ve1.1 or Ve1.2, and then inoculated with V. albo-atrum, exhibited reduced and/or delayed symptoms of verticillium wilt (e.g. wilting, chlorosis, necrosis) relative to inoculated control plants transformed only with vector DNA. As shown in Table 1, untransformed potato plants (Dxc3xa9sirxc3xa9ee), or potato plants transformed only with vector DNA (pBI121 and pBIN19) exhibited no resistance to V. albo-atrum. In contrast, plants transformed with the Ve1.1 genomic and cDNA sequences (pG3Ve and pC1.1Ve, respectively) and Ve1.2 genomic and cDNA sequences (pG2Ve and pC1.2Ve, respectively) exhibited substantial resistance to V. albo-atrum. Hence, not only do the exemplified Ve polynucleotides confer resistance on various plant species (tomato, potato), they confer resistance to various Verticillium species (V. dahliae and V. albo-atrum).
In another exemplified case, the Ve polynucleotide is isolated from Solanum chacoense, a wild potato variety, and has a partial nucleotide sequence as depicted in SEQ ID NO: 13.
It will be appreciated by those of skill in the art that, due to the degeneracy of the genetic code, numerous functionally equivalent nucleotide sequences encode the same amino acid sequence. Therefore, all Ve polynucleotides that encode the Ve polypeptides depicted in SEQ ID NOS: 2, 4, 6, and 8, and the partial Ve polypeptide sequence depicted in SEQ ID NO: 14, are included in the invention.
Further, strains of L. esculentum or S. chacoense may contain naturally occurring allelic variants of the exemplified Ve polynucleotides. All such allelic variants of the exemplified Ve polynucleotides Ve1.1 and Ve1.2, and the exemplified partial Vc sequence, and the encoded Ve polypeptides are included within the scope of the invention.
The invention also extends to truncated Ve polynucleotides and Ve polypeptides that, despite truncation, retain the ability to confer on plants resistance to Verticillium species. For instance, as discussed in Example 1 herein, a fragment of Ve1.1, 1332 nucleotides in length (SEQ ID NO: 1 from nucleotide 1 to nucleotide 1332), and a fragment of Ve1.2, 1146 nucleotides in length (SEQ ID NO: 5 from nucleotide 1 to nucleotide 1146), were both were functional to confer on a plant resistance to Verticillium species.
Using a variety of techniques that are well known in the art (generally as described in Sambrook et al., 1989; Ausubel et al., 1990; Ausubel et al., 1999), the exemplified Ve polynucleotides can be used to isolate additional Ve polynucleotides. Ve polynucleotides of the invention can be isolated from, without limitation, tomato, potato, hop, alfalfa, strawberry, sainfoin, runner bean, broad bean, pea, clover, cucumber, canola, cotton, dahlia, mint species, vine, eggplant, olive, pistachio, stone fruit, Brussels sprouts, groundnut, horse radish, tobacco, and red pepper. In particular, Ve polynucleotides can be isolated from Solanaceous plants. For instance, as discussed in detail in Example 2 herein, the inventors have now mapped a highly active Verticillium wilt resistance gene Vc in Solanum chacoense, a wild potato variety, to the same position on chromosome IX as Ve1 occurs in tomato. Further, complete linkage between Vc and Verticillium wilt resistance was observed, and Vc hybridized with a probe comprising a portion of Ve1.1. Moreover, sequence alignment analysis of the Ve1.2 deduced amino acid sequence and a partial deduced amino acid sequence of Vc, having 327 amino acid residues, revealed 72.48% sequence identity. Hence, it appears that Vc and Ve1 may be related Ve polynucleotides.
The Ve polynucleotides depicted in SEQ ID NOS: 1, 3, 5 and 7, and the partial Ve polynucleotide sequence depicted in SEQ ID NO: 13 can be used to construct probes for use in nucleic acid hybridization assays with genomic DNA or complementary DNA (xe2x80x9ccDNAxe2x80x9d) libraries to identify homologous nucleic acid sequences. The principle of hybridization analysis is that a single-stranded DNA or RNA molecule, having a defined sequence, can base-pair with a second DNA or RNA molecule having a complementary sequence to the probe, and which is immobilized. The stability of the probe/target sequence hybrid is dependent on the extent of base pairing that occurs. Techniques for constructing primers and probes, for making recombinant DNA libraries, and for performing nucleic acid hybridization, are well known in the art.
In order to construct a probe, the exemplified Ve polynucleotides can be used first to design a pair of primers. Alternately, degenerate primers may be designed based on the exemplified Ve polypeptides (SEQ ID NOS: 2, 4, 6 and 8), or the exemplified partial Ve polypeptide (Vc) depicted in SEQ ID NO: 14. The primers are typically obtained by using chemical DNA synthesis to form oligonucleotides of about 30 nucleotides. The primers may be based on any part of the exemplified Ve polynucleotides or Ve polypeptides. Known sequence alignment techniques, as described hereinbefore, can be used to identify conserved regions, which may be preferred sources for primers.
The primers can then used to amplify by polymerase chain reaction (xe2x80x9cPCRxe2x80x9d) a homologous sequence from polynucleotides isolated from a plant that may be either susceptible or resistant to Verticillium species. The plant may conveniently be a resistant tomato strain as exemplified herein. The amplified sequence is obtained for subsequent use as a probe in a hybridization assay. Hybridization probes generally have a minimum length of about 200 nucleotides, although smaller probes (e.g. as little as 50 nucleotides in length) can also be used.
To obtain the probe by PCR, double stranded DNA to be amplified is denatured by heating. In the presence of DNA polymerase, excess deoxyribonucleoside triphosphates (xe2x80x9cdNTPsxe2x80x9d), buffers, salts, and excess single-stranded oligonucleotide primers (based on the exemplified Ve polynucleotides or Ve polypeptides), new DNA synthesis occurs. The primers hybridize to opposite strands of the DNA, and DNA polymerase catalyzes the extension of new strands in the 5xe2x80x2 to 3xe2x80x2 direction across the DNA segment bounded by the primers. The first cycle of synthesis results in new strands of indeterminate length which, like the parental strands, can hybridize to the primers in the next cycle of denaturation and annealing. These products of indeterminate length build up only arithmetically during subsequent cycles of denaturation, annealing, and DNA synthesis. But discrete products (the target sequence), which are the length of the portion of the parental strands between the 5xe2x80x2 ends of the primers, accumulate at an exponential rate, doubling in each cycle of denaturation, annealing and DNA synthesis. Typically, between 20 and 40 cycles are used.
The amplified probe sequence may then be purified by gel electrophoresis. Alternatively, the probe sequence can be cloned into a plasmid and maintained therein, then restricted out of the plasmid and purified by gel electrophoresis. The probe is typically then labeled by, for instance, radio-labeling or biotin-labeling, to permit ready visualization.
The probes based on the exemplified Ve polynucleotide or Ve polypeptide sequences can be used to probe a genetic library of a resistant plant strain. As discussed earlier, a wide variety of plant species may be used as the source of the genetic library. Any resistant strain of a plant species that is known to be affected by Verticillium wilt is a likely candidate for the isolation of Ve polynucleotides. Libraries may be obtained from commercial sources or constructed by known techniques. Genomic libraries are generally constructed by digesting genomic DNA to fragments of manageable size using restriction endonucleases, packaging the genomic DNA fragments into vectors such as bacteriophage xcex vectors or cosmid vectors, and introducing the recombinant vectors into suitable host cells (generally E. coli). Sufficient numbers of clones are generated to ensure that the particular sequence of interest is represented. The construction of cDNA libraries is similar, but commences with the generation of a double-stranded DNA copy of messenger RNA (xe2x80x9cmRNAxe2x80x9d) from plant tissues of interest through reverse transcription. A primer is annealed to the mRNA, providing a free 3xe2x80x2 end that can be used for extension by the enzyme reverse transcriptase. The enzyme engages in the usual 5xe2x80x2-3xe2x80x2 elongation, as directed by complementary base pairing with the mRNA template to form a hybrid molecule, consisting of a template RNA strand base-paired with the complementary cDNA strand. After degradation of the original mRNA, a DNA polymerase is used to synthesize the complementary DNA strand to convert the single-stranded cDNA into a duplex DNA.
In a common approach to hybridization analysis, once the appropriate library is constructed, the library is plated out, transferred to a solid support membrane such as a nitrocellulose filter or nylon membrane, and hybridized to the labeled probe.
All hybridization methods (discussed in detail in Sambrook et al., 1989; and in Ausubel et al., 1990, 1999) depend on the ability of denatured DNA to re-anneal when complementary strands are present in an environment near, but below, their melting temperature (Tm), the temperature at which fifty percent of existing DNA duplex molecules are dissociated into single strands. A number of annealing reactions occur during hybridization. These include: annealing of the probe to homologous DNA sequences; mis-matched annealing of the probe to partially homologous sequences; and non-sequence specific interactions, which result in background noise. Mis-matched sequences form less stable hybrids than do completely homologous sequences. As a general rule, the Tm of a double-stranded DNA molecule decreases by 1-1.5xc2x0 C. with every 1% decrease in homology. Increases in temperature, and decreases in salt concentration disfavour annealing, and increase the stringency of the assay. Therefore, hybridization and wash conditions can be adjusted to achieve desired levels of annealing.
Hybridization is typically carried out in solutions of high ionic strength (e.g. 6xc3x97SSC (sodium chloride/sodium citrate buffer) or 6xc3x97SSPE (20xc3x97SSPE=3.0 M NaCl, 0.2 M NaH2PO4.H2O, 20 mM EDTA, pH 7.4)) at a temperature 20-25 xc2x0 C. below Tm. For Na+ concentrations in the range of 0.01 M to 0.4 M, and G+C content from about 30-70%, Tm of hybrids of greater than 100 nucleotides in length can be estimated by the equation Tm=81.5xc2x0 C.xe2x88x9216.6(log10[Na+])+0.41(%G+C)xe2x88x920.63(% formamide)xe2x88x92(600/1), where 1=the length of the hybrid in base pairs. This equation applies to the xe2x80x9creversiblexe2x80x9d Tm defined by measurement of hyperchromicity at OD257. The xe2x80x9cirreversiblexe2x80x9d Tm, which is more important for autoradiographic detection of DNA hybrids is usually 7-10xc2x0 C. higher. (Sambrook et al., 1989). A convenient formula for estimating hybridization temperature (Th) provided in product literature for NYTRAN brand nylon membranes is Th=Tmxe2x88x925xc2x0 C.=2xc2x0 C. (Axe2x88x92T bp)+4xc2x0 C. (Gxe2x88x92C bp)xe2x88x925xc2x0 C. To avoid background problems, hybridization time and the amount of probe used should be minimized. The probe preferably has a high specific activity and a length of at least about 50 nucleotides.
Washing is performed to remove excess probe, as well as probes that are bound as mis-match hybrids having less than a desired homology level. Washing proceeds in the order from least stringent to most stringent conditions. The stringency of the wash conditions can be varied by adjusting the temperature and salt concentrations of the wash solution. These conditions can be determined empirically by preliminary experiments in which samples of the DNA to be probed are immobilized on filters, hybridized to the probe, and then washed under conditions of different stringencies. By way of illustration, a typical low stringency wash may be conducted at room temperature in a solution of 2xc3x97SSC and 0.1% SDS (sodium dodecyl sulfate). A typical high stringency wash may be conducted at 68xc2x0 C. in a solution of 0.1xc3x97SSC and 0.5% SDS.
Clones that hybridize with the probe at the desired stringency level can then be rescued or isolated, and then sequenced, again using known techniques. As discussed earlier, comparison of the newly isolated sequence with the exemplified Ve polynucleotide sequences can be performed visually, or by using known algorithms and software packages. Those sequences exhibiting at least 40% homology to the exemplified Ve polynucleotides can then be tested to determine whether they are functional to confer on a susceptible plant resistance to Verticillium species. A suitable test is the complementation test described in Example 1herein. Cells or tissues of a susceptible plant are transformed with the polynucleotide of interest (e.g. as described by De Block, 1988) and transformed plants regenerated. The plants are inoculated with a strain of Verticillium which causes Verticillium wilt in the susceptible plant variety. The inoculated plants are examined over an appropriate time course (e.g. at four weeks after inoculation, then at weekly intervals for an additional four weeks) for symptoms of Verticillium wilt. Preferably, control plants such as susceptible non-transformed plants or susceptible plants transformed only with vector DNA are also inoculated with the disease-causing Verticillium strain and! then monitored for disease symptoms. Delayed or reduced disease symptoms are indicative that the isolated polynucleotide comprises a functional Ve polynucleotide.
The exemplified Ve polynucleotides and Ve polypeptides can be used in conjunction with other known techniques to obtain Ve polynucleotides from genomic DNA, cDNA, RNA, proteins, sequence databases, or plant cells or tissues. For example, using primers derived from the exemplified Ve1.1 or Ve1.2 sequences, Ve polynucleotides can be amplified from genomic DNA, cDNA or genomic or cDNA libraries of tomato plants or other plant species (Leister et al., 1996).
In a further alternative method, known immunodetection techniques employing antibodies specific to the Ve1.1 and Ve1.2 polypeptides, or the partial Vc polypeptide, can be used to screen plant cells, tissues, or extracted proteins of interest for the presence of related Ve polypeptides (Sambrook et al., 1989).
Again, such sequences can then be tested by the complementation tests described in Example 1 herein, to determine whether they function to confer on plants resistance to Verticillium species.
Additionally, those of skill in the art, through standard mutagenesis techniques, in conjunction with the complementation tests described in Example 1 herein, can obtain altered Ve polynucleotides and test them for the property of conferring, on plants resistance to Verticillium species. Useful mutagenesis techniques known in the art include, without limitation, oligonucleotide-directed mutagenesis, region-specific mutagenesis, linker-scanning mutagenesis, and site-directed mutagenesis by PCR (see e.g. Sambrook et al., 1989 and Ausubel et al., 1990, 1999).
In obtaining, variant Ve polynucleotides, those of ordinary skill in the art will recognize that proteins may be modified by certain amino acid substitutions, additions, deletions, and post-translational modifications, without loss or reduction of biological activity. In particular, it is well-known that conservative amino acid substitutions, that is, substitution of one amino acid for another amino acid of similar size, charge, polarity and conformation, are unlikely to significantly alter protein function. The 20 standard amino acids that are the constituents of proteins can be broadly categorized into four groups of conservative amino acids on the basis of the polarity of their side chains R- groups) as follows: the nonpolar (hydrophobic) group includes alanine, isoleucine, leucine, methionine, phenylalanine, proline, tryptophan and valine; the polar (uncharged, neutral) group includes asparagine, cysteine, glutamine, glycine, serine, threonine and tyrosine; the positively charged (basic) group contains arginine, histidine and lysine; and the negatively charged (acidic) group contains aspartic acid and glutamic acid. Substitution in a protein of one amino acid for another within the same group is unlikely to have an adverse effect on the biological activity of the protein.
As shown in FIG. 9, Ve1.1 and Ve1.2, while both are functional to confer on plants resistance to Verticillium species, have sequence identity or homology of only about 84%. Those amino acids that are not identical are likely not essential to protein function. Hence, variation at these amino acids likely will not negatively affect the biological activity of Ve1.2 or Ve1.2. Further, those amino acids that are not identical, but that are similar, likely can be replaced by other similar amino acids, as discussed in the preceding paragraph, without loss of function.
Exemplified Ve polynucleotides isolated from a strain of Lycopersicon esculentum which is resistant to Verticillium dahliae have been transformed into Solanum tuberosum strains that are susceptible to infection by V. albo-atrum, resulting in functional complementation. The transformed Solanum tuberosum strains displayed reduced disease symptoms following inoculation with V. albo-atrum relative to non-transformed plants, illustrating that Ve polynucleotides of the invention are useful for conferring resistance to various Verticillium species on a range of plants. Ve polynucleotides of the invention can also be used to confer resistance to Verticillium species in all higher plants which are susceptible to infection by Verticillium species, including, without limitation, tomato, potato, hop, alfalfa, strawberry, sainfoin, runner bean, broad bean, pea, clover, cucumber, canola, cotton, dahlia, mint species, vine, eggplant, olive, pistachio, stone fruit, Brussels sprouts, groundnut, horse radish, tobacco, and red pepper.
In preparation for transformation of plant cells with Ve polynucleotides, recombinant vectors are prepared. The desired recombinant vector generally comprises an expression cassette designed for initiating transcription of the Ve polynucleotide in the transformed plant. Additional sequences are included to allow the vector to be cloned in a bacterial or phage host.
The vector will preferably contain a prokaryotic origin of replication having a broad host range. A selectable marker should also be included to allow selection of bacterial cells bearing the desired construct. Suitable prokaryotic selectable markers include resistance to antibiotics such as ampicillin.
Other DNA sequences encoding additional functions may also be present in the vector, as is known in the art. For instance, in the case of Agrobacterium mediated transformation, T-DNA sequences will also be included for subsequent transfer to plant chromosomes.
For expression in plants, the recombinant expression cassette will preferably contain, in addition to the desired sequence, a plant promoter region, a transcription initiation site (if the sequence to be transcribed lacks one), and a transcription termination sequence. Unique restriction enzyme sites at the 5xe2x80x2 and 3xe2x80x2 ends of the cassette are typically included to allow for easy insertion into a pre-existing vector. Sequences controlling eukaryotic gene expression are well known in the art.
The particular promoter used in the expression cassette is not critical to the invention. Any of a number of promoters which direct transcription in plant cells is suitable. The promoter can be either constitutive or inducible. A number of promoters which are active in plant cells have been described in the literature. These include the nopaline synthase (NOS) and octopine synthase (OCS) promoters (which are carried on tumour-inducing plasmids of Agrobacterium tumefaciens), the caulimovirus promoters such as the cauliflower mosaic virus (CaMV) 19S and 35S and the figwort mosaic virus 35S-promoters, the light-inducible promoter from the small subunit of ribulose-1,5-bis-phosphate carboxylase (ssRUBISCO, a very abundant plant polypeptide), and the chlorophyll a/b binding protein gene promoter. All of these promoters have been used to create various types of DNA constructs which have been expressed in plants. A particularly preferred promoter is the endogenous promoter of the Ve polynucleotide. For instance, with respect to Ve1.1 and Ve1.2, the endogenous promoters in SEQ ID NO: 10, are particularly preferred.
The entirety of the endogenous promoter region depicted in SEQ ID NO: 10 need not be used. A relatively short sequence within SEQ ID NO: 10, of as little as 50 nucleotides, may be functional as a promoter, provided that the elements essential to promoter function are included. The promoter region contains sequence of bases that signals RNA polymerase to associate with the DNA, and to initiate the transcription of mRNA using one of the DNA strands as a template to make a corresponding complimentary strand of RNA. Promoter sequence elements include the TATA box consensus sequence (TATAAT), which is usually 20 to 30 base pairs (bp) upstream (by convention xe2x88x9230 to xe2x88x9220 bp relative to the transcription start site) of the transcription start site. In most instances the TATA box is required for accurate transcription initiation: The TATA box is the only upstream promoter element that has a relatively fixed location with respect to the start point. The CAAT box consensus sequence is centered at xe2x88x9275, but can function at distances that vary considerably from the start point and in either orientation. Hence, the TATA box and CAAT box may be within 50 nucleotides of each other. Another common promoter element is the GC box at xe2x88x9290 which contains the consensus sequence GGGCGG. It may occur in multiple copies and in either orientation. Other sequences conferring tissue specificity, response to environmental signals, or maximum efficiency of transcription may also be found in the promoter region. Such sequences are often found within 400 bp of transcription initiation size, but may extend as far as 2000 bp or more. In heterologous promoter/structural gene combinations, the promoter is preferably positioned about the same distance from-the heterologous transcription start site as it is from the transcription start site in its natural setting. However, some variation in this distance can be accommodated without loss of promoter function.
Sequences within SEQ ID NO: 10 that provide promoter function can be readily identified. A chimeric construct is created, which includes the fragment of SEQ ID NO: 10 to be tested for promoter function, operably linked to a reporter gene. Protoplasts are transformed with the chimeric construct, and the expression of the reporter gene is measured. High expression of the reporter gene is indicative of strong promoter function. A suitable reporter gene for the analysis of plant gene expression is the bacterial gene uidA, encoding xcex2-glucuronidase (xe2x80x9cGUSxe2x80x9d). GUS expression can be conveniently quantified through a highly sensitive non-radioactive assay using the fluorogenic substrate 4-MUGluc, as described by Gelvin et al. (1994).
In addition to a promoter sequence, the expression cassette preferably also contains a transcription termination region downstream of the structural gene to provide for efficient termination. The termination region may be obtained from the same gene as the promoter sequence, from a different gene, or may be the endogenous termination region of the Ve polynucleotide.
Polyadenylation sequences are also commonly added to the vector construct if the mRNA encoded by the structural gene is to be efficiently translated (Alber et al., 1982). Polyadenylation is believed to have an effect on stabilizing mRNAs. Polyadenylation sequences include, but are not limited to the Agrobacterium octopine synthase signal (Gielen et al., 1984) or the nopaline synthase signal (Depicker et al., 1982).
The vector will also typically contain a selectable marker gene by which transformed plant cells can be identified in culture. Typically, the marker gene encodes antibiotic resistance. These markers include resistance to G418, hygromycin, bleomycin, kanamycin, and gentamycin. After transforming the plant cells, those cells containing the vector will be identified by their ability to grow in a medium containing the particular antibiotic.
The recombinant vector is assembled by employing known recombinant DNA techniques (Sambrook et al., 1989; Ausubel et al., 1990, 1999). Restriction enzyme digestion and ligation are the basic steps employed to join two fragments of DNA. The ends of the DNA fragment may require modification prior to ligation, and this may be accomplished by filling in overhangs, deleting terminal portions of the fragment(s) with nucleases (e.g., ExoIII), site directed mutagenesis, or by adding new base pairs by PCR. Polylinkers and adaptors may be employed to facilitate joining of selected fragments. The recombinant vector is typically assembled in stages employing rounds of restriction, ligation, and transformation of E. coli. 
Recombinant vectors can be introduced into plant cells by a variety of known techniques. Although in the exemplified case potato plant stem and leaf sections were transformed via inoculation with Agrobacterium tumefaciens carrying the Ve polynucleotide sequence linked to a binary vector, direct transformation techniques which are known in the art can also be used to transfer the recombinant DNA. For instance, the vector can be microinjected directly into plant cells. Alternatively, nucleic acids may be introduced to the plant cell by high velocity ballistic penetration by small particles having the nucleic acid of interest embedded within the matrix of the particles or on the surface. Fusion of protoplasts with lipid-surfaced bodies such as minicells, cells or lysosomes carrying the DNA of interest can be used. The DNA may also be introduced into plant cells by electroporation, wherein plant protoplasts are electroporated in the presence of plasmids carrying the expression cassette, or by polyethylene glycol (xe2x80x9cPEGxe2x80x9d)-mediated transformation. A review of some of the techniques for incorporating foreign DNA into plant cells is found in Gelvin et al. (1994).
In contrast to direct transformation methods, the exemplified case involves vectored transformation using Agrobacterium tumefaciens. Agrobacterium tumefaciens is a Gram-negative soil bacteria which causes a neoplastic disease known as crown gall in dicotyledonous plants. Induction of tumours is caused by tumour-inducing plasmids known as Ti plasmids. Ti plasmids direct the synthesis of opines in the infected plant. The opines are used as a source of carbon-and/or nitrogen by the Agrobacteria.
The bacterium does not enter the plant cell, but transfers only part of the Ti plasmid, a portion called T-DNA, which is stably integrated into the plant genome, where it expresses the functions needed to synthesize opines and to transform the plant cell. Vir (virulence) genes on the Ti plasmid, outside of the T-DNA region, are necessary for the transfer of the T-DNA. The vir region, however, is not transferred. In fact, the vir region, although required for T-DNA transfer, need not be physically linked to the T-DNA and may be provided on a separate plasmid.
The tumour-inducing portions of the T-DNA can be interrupted or deleted without loss of the transfer and integration functions, such that normal and healthy transformed plant cells may be produced which have lost all properties of tumour cells, but still harbour and express certain parts of T-DNA, particularly the T-DNA border regions. Therefore, modified Ti plasmids, in which the disease causing genes have been deleted, may be used as vectors for the transfer of the Ve polynucleotide constructs of the present invention into plants.
Transformation of plants cells with Agrobacterium and regeneration of whole plants typically involves either co-cultivation of Agrobacterium with cultured isolated protoplasts or transformation of intact cells or tissues with Agrobacterium. In the exemplified case, potato stem and leaf sections were transformed with Agrobacterium.
Alternatively, cauliflower mosaic virus (CaMV) may be used as a vector for introducing DNA into plants of the Solanaceae family. For instance, U.S. Pat. No. 4,407,956 to Howell teaches the use of cauliflower mosaic virus DNA as a plant vehicle.
After transformation, transformed plant cells or plants carrying the recombinant DNA are identified. A selectable marker, such as antibiotic resistance, is typically used. In the exemplified case, transformed plant cells were selected by growing the cells on growth medium containing carbenicillin and kanamycin. Other selectable markers will be apparent to those skilled in the art. For instance, the presence of opines can be used to identify transformants if the plants are transformed with Agrobacterium.
Expression of the foreign DNA can be confirmed by detection of RNA encoded by the inserted DNA using well known methods such as Northern blot hybridization. The inserted DNA sequence can itself be identified by Southern blot hybridization or by PCR (see, generally, Sambrook et al., 1989).
Generally, after it is determined that the transformed plant cells carry the recombinant DNA, whole plants- are regenerated. Techniques for regenerating differentiated transgenic plants from-transformed cells are well known in the art and are described in detail in such references as Gelvin et al. (1994). In the exemplified case, potato stem and leaf sections were inoculated with a culture of Agrobacterium tumefaciens carrying the desired Ve polynucleotide and carbenicillin and kanamycin marker genes. Transformants were selected on a growth medium containing carbenicillin and kanamycin. After transfer to a suitable medium for shoot induction, shoots were transferred to a medium suitable for rooting. Plants were then transferred to soil and hardened off. The plants regenerated in culture were transplanted and grown to maturity under greenhouse conditions.
The resistance of the regenerated transgenic plants to Verticillium species may then be tested. An aggressive isolate of a Verticillium wilt-causing pathogen such as Verticillium albo-atrum is isolated from infected plants, and cultured. Transgenic plants containing the Ve polynucleotide and control plants are then inoculated with a spore suspension of the cultured pathogen. The control plants may be wild type Verticillium-sensitive plants, or transgenic plants transformed only with vector DNA which does not contain the Ve polynucleotide insert. Disease reaction ratings can be measured visually, over an appropriate time course. For instance, in potato plants, the first observations may be made four weeks after inoculation, and conducted thereafter at weekly intervals for another four weeks. Plants that show delayed or reduced disease symptoms such as wilting, chlorosis, and necrosis of the leaves, relative to control plants, are rated as being resistant to Verticillium species.
Recombinant DNA procedures used for practicing the invention and which are not described in detail herein involve standard laboratory techniques that are well known in the art and are described in standard references such as Sambrook et al. (1989) or Ausubel et al. (1990, 1999). Generally, enzymatic reactions involving DNA ligase, DNA polymerase, restriction endonucleases and the like are performed according to the manufacturer""s specifications. Abbreviations and nomenclature employed herein are standard in the art and are commonly used in scientific publications such as those cited herein.